1. Field of the Invention
This invention relates to a new or improved tool for installing piles in underwater ground (i.e. soil or rock) formations, and to a method for utilizing such a tool.
2. Description of the Prior Art
Offshore structures, vessels and floating rigs require anchoring points to safely moor or position them in situ, or to offer resistance to allow rig repositioning or movement. Known anchoring systems use fluke, gravity or suction type anchors or driven foundation piles to provide resistance to lateral or tension loads. Fluke, gravity and suction anchors provide limited and unquantified pull-out resistance, and have been installed in various ways, e.g. by dragging flukes into the sea bed, or by suctioning structures into the sea bed. A notable disadvantage of these anchors is the fact that they are only effective to take up horizontal forces so that the horizontal spacing between the anchor points and the structure that is being anchored needs to be very large. The present invention overcomes this disadvantage, so that there results a large saving in the costs for anchoring ropes or cables.
Efforts have been made to use explosives, pneumatic and hydraulically powered systems to shoot anchors into the sea bed, examples of these being seen in U.S. Pat. Nos. 3,170,433 Gardiner, 4,619,218 Kenny and 4,682,559 Schnitzer et al.
Shallow water systems may make use of piles driven and/or drilled into the ocean floor to provide resistance to compression, tension, or lateral loads, which they can do effectively. However in deep sea and ultra-deep environments, increased installation costs limit the use of subsea pile driving hammers for piled foundations. As is well understood, a pile driver system operates through the repeated striking of a foundation element with blows or forces of high magnitude, thus advancing the foundation element into the ground in increments. The kinetic energy output of a pile driver is a function of its ram mass and the velocity of the ram at impact. Pile driving is accomplished through transmission of the kinetic energy of the pile driver to the pile to overcome resistance and loss forces and impart a displacement to the pile.
Conventional inland pile drivers do not operate efficiently under water, and for such applications specialized underwater pile drivers have been developed, examples of these being seen in U.S. Pat. Nos. 4,238,166 Gendron and 4,362,439 Vaynkof.
It is the object of the present invention to provide a novel method and apparatus for the installation of piles and other types of foundation elements into the sea bed. Desirably the system will be portable so that it can be readily transferred between different locations for the installation of foundation elements. Also within the scope of the invention drive systems can be developed which are compatible with highly inflammable environments.
The invention provides a tool for use in submerged condition for installing piles and other types of foundation elements into a ground formation that is submerged under a body of water, comprising: a hammer body that is adapted to be fixedly supported relative to and in axial alignment with the head of a pile that is to be driven; a reaction body carried by said hammer body and guided for movement thereon in a direction that is axial to the pile that is to be driven; said hammer body and said reaction body respectively defining opposed first and second ends of an expansion chamber that is formed therebetween; charging means for creating a rapidly expanding volume of high pressure gas within said expansion chamber to generate a downwards pressure force pulse on said expansion chamber first end to drive the pile, an equal and opposite upwards pressure force pulse being applied to said reaction body through said second end of the expansion chamber; and damping structure operatively associated with said reaction body and configured to interact with the water in which the tool is submerged to resist upwards movement of said reaction body in response to such upwards pressure force pulse.
Preferably the charging means comprises a series of combustible propellant charges each arranged within a firing chamber which communicates with the expansion chamber through a connecting passage to deliver high pressure gas to the expansion chamber upon initiation of the respective charge. The charging chambers can be arranged in a housing that surrounds the reaction body, each firing chamber communicating with the expansion chamber through a non-return valve. A fuel igniter in each firing chamber is connected to an igniter control on the tool, and the igniter control is arranged for remote actuation, e.g. through a cable leading to the surface or to a WROV (working remote operated vehicle), or by wireless arrangements involving radio frequency waves.
The damping structure is preferably a large volume container that has an open top and that is positioned on the reaction body. The container itself although large can be thin-walled and relatively lightweight, but will enclose a very large mass of water the inertia of which is used to resist upward displacement of the reaction body. The bottom of the container preferably has a series of valve ports extending upwardly therethrough each valve port having a valve closure mounted to permit flow of water upwardly into the container, but to prevent flow of water downwardly out of the container. Thus the valve arrangement allows the container to settle downwards again rapidly, after an upwards displacement in response to firing of a propellant charge.
It will be understood that the pressure pulses provide a repeatable downwards thrust or push upon the pile, and that this thrust is generated by a mechanism which does not require any ram or movable striking or oscillating part or other mechanism to transfer kinetic energy to the foundation element. Rather the thrust is created through the pressurized gas acting downwardly onto the pile, this gas being contained within the tool mechanism which remains a separate entity from the foundation element or pile that is being installed.
The hammer body and the reaction body together provide a mated and guided piston and cylinder pressure vessel, the parts of which remain connected throughout use, although being free for axial movement relative to one another.
The direction of the applied load to the foundation element is easily determined and controlled through positioning and alignment of the tool, and although for convenience of description the terms "upwardly" and "downwardly" and the like are employed herein it will be understood that the disclosed method and apparatus is not restricted to the driving of foundation elements vertically, but is also useful where the foundation elements are to be installed in angled or even horizontal orientations.
The composition and size of each propellant charge can be adjusted as desired to provide the desired impulse shape (as to duration and magnitude) best suited to the geotechnical conditions at hand.
Resistance to the upward reaction force is provided by the mass of water which is within the container and which provides a combined inertial and drag resistance to the acceleration and motion of the contained mass through the water. Motion of this resistance system is designed and desired to occur as a consequence of the large reaction thrust load. The magnitude of the motion is intended to be high such that during the thrust application the resistance system is accelerated through the water, in vertical installations the container being restored to its start position under the force of gravity.
Angled deployment of foundation elements such as piles may be obtained by ballasting and similar controlled initial penetration of the foundation element. The desired inertial and drag resistance of the container will still be achieved even when angled, but in such applications some force mechanism such as spring means may be required to restore the container to its starting position.
The tool is readily adaptable to include electronic transducer systems to measure load and position (displacement) of the foundation element over time, i.e. prior to, during, and after each successive thrust. Continuous monitoring, recording and analysis of the applied thrust loads and foundation element advance is provided remotely from the operating station. Thus a complete foundation penetration record can be provided which gives high quality assurance and certification of ultimate attained foundation capacity and stiffness. Due to such certification, piles can also be installed at places where only limited soils information is available. In other words less soils information is needed to ensure that a safe and acceptable anchoring point or foundation pile is achieved.